When Al Gore said last week that scientists now have clear proof that climate change is directly responsible for the extreme and devastating floods, storms and droughts that displaced millions of people this year, my heart sank. Having suggested the idea of "event attribution" back in 2003, and co-authored a study published earlier this year on the origins of the UK floods in autumn 2000, I suspect I may be one of the scientists being talked about.
Gore is right that it is possible, in principle, to quantify the role of human influence on climate in specific weather events, and that this has to involve probability: how much has human influence "loaded the weather dice" to make a particular event more likely? Such questions can be answered, and because the impacts of climate change are overwhelmingly felt through changing risks of extreme weather, the answers matter. People deserve to know how much climate change is affecting them, and not be fobbed off with banalities like: "this is the kind of event that we might expect to become more frequent."
But the fact that a method exists for establishing whether or not a statement is true does not mean that it is true, still less that anyone has done the study to find out. To my knowledge, formal probabilistic attribution analyses have only been published on two specific events: the 2003 European heatwave and the autumn 2000 UK floods. Both studies found human influence on climate had most likely increased the risk of the event in question, but in the case of the autumn 2000 floods we found a one in 10 chance that the increase was a modest 20% or less. And a follow-up study, just published in the Journal of Hydrology by Alison Kay and co-authors, used the same data to look at factors affecting the risk of a hypothetical flood in spring 2001. They found that greenhouse gas emissions had actually reduced the risk of such a flood: understandably, since springtime floods in the UK tend to result from melting snow, and thanks to greenhouse warming there is now less snow around.
This illustrates an important point: human influence on climate is making some events more likely, and some less likely, and it is a challenging scientific question to work out which are which. Randy Dole and co-workers found no evidence for human influence increasing the risk of the 2010 Russian heatwave, the jury is still out on the Pakistani floods, and has broken up in disarray over hurricane Katrina. So when Gore says: "the environment in which all storms are formed has changed," he isn't actually lying, but he is begging to be misunderstood.
The claim that we are "painting more dots on the dice", causing weather events that simply could not have occurred in the absence of human influence on climate, is just plain wrong. Given the paucity of reliable records and bias in climate models, it is quite impossible to say whether an observed event could have happened in a hypothetical pristine climate. Our research focuses on quantifying how risks have changed, which is a much easier proposition, although addressing all the uncertainties still makes working out these "relative risks" a painstaking affair.
Enthusiasm for doing anything about climate change seems to have given way to resignation that we will simply have to adapt. For the foreseeable future, this overwhelmingly means dealing with harmful weather events that have been made more likely by human influence on climate. What we can't say right now is which these events are, and therefore who is being harmed and how much.
But this question can be answered: in principle, using exactly the same models that are used for weather forecasting, not the much-derided low-resolution variants that are used to predict the climate of 2200. And it deserves to be answered properly: the autumn 2000 flood study took us five years and tens of thousands of detailed simulations, all performed using computing capacity kindly donated by the public. We're hoping to get a bit quicker off the mark in future, but it is frustrating when Gore claims to know the answer before we have even asked the question.
• This article was amended on 13 October 2011 to remove the quotation marks in the first sentence. Myles Allen was quoting the linked article, not Al Gore.